The mechanism of carcinogenesis by polycyclic hydrocarbons is one of the important unsolved problems of cancer research. Controversial evidence points to the occurrence of active olefinic "K-regions" that are primarily responsible for the carcinogenetic potential of certain benzenoid compounds. The synthesis of polycyclic hydrocarbons fused to or incorporating activating rings is proposed, to enable the study of the physical and chemical properties of these novel compounds. The activating ability of such rings will be related to their strain and electronic make up. In larger systems, fusion will be designed such as to activate remote K-regions of known carcinogens. Novel synthetic routes developed in our laboratory will be applied to attain this goal. The new hydrocarbons will be made available for physiological testing and arrangements have been made to this effect. The carcinogenic and mutagenic potential of these new compounds will be compared with the activity of the known parent compounds.